Prenatal cranial ossification of the humpback whale (Megaptera novaeangliae)

Being descendants of small terrestrial ungulate mammals, whales underwent enormous transformations during their evolutionary history, that is, extensive changes in anatomy, physiology, and behavior were evolved during secondary adaptations to life in water. However, still only little is known about whale ontogenetic development, which help to identify the timing and sequence of critical evolutionary events, such as modification of the cetacean ear. This is particularly true for baleen whales (Mysticeti), the group including the humpback whale Megaptera novaeangliae. We use high-resolution X-ray computed tomography to reinvestigate humpback whale fetuses from the Kükenthal collection at the Museum für Naturkunde, Berlin, thus, extending historic descriptions of their skeletogenesis and providing for the first time sequences of cranial ossification for this species. Principally, the ossification sequence of prenatal Megaptera follows a typical mammalian pattern with the anterior dermal bones being the first ossifying elements in the skull, starting with the dentary. In contrast to other mammals, the ectotympanic bone ossifies at an early stage. Alveolar structure can be observed in both the maxillae and dentaries in these early prenatal specimens but evidence for teeth is lacking. Although the possibility of obtaining new embryological material is unlikely due to conservation issues, our study shows that reexamination of existing specimens employing new technologies still holds promise for filling gaps in our knowledge of whale evolution and ontogeny.